1. Field of the Invention
The present invention relates to an electrostatic charge image developer for use in developing an electrostatic charge images formed by electrophotography or by electrostatic recording.
2. Description of the Related Art
Various methods for visualizing image information as electrostatic charge images, such as electrophotography, are now widely used in various fields. In electrophotography, an electrostatic latent image is formed on a photoreceptor in two stages, a charging stage and in an exposure stage. The electrostatic latent image is then developed using a developer including a toner and is visualized through transfer and fixation stages. Developers used for this purpose include a two component developer comprising a toner and a carrier and a single component developer used as a single entity, such as a magnetic toner. Of these, the two component developers are widely in use because of advantages such as, for example, that, in the two component developers, the functionalities as the developer are separated as the carrier performs the functions of stirring, transfer, charging, etc. of the developer and that the developer does not include a magnetic powder which is advantageous in color images because the coloring is superior.
In general, toners are manufactured through a mixing and grinding method which includes the steps of molten dispersion of a thermoplastic resin with a pigment, a charge control agent, and a release agent such as wax; cooling; fine grinding; and classifying. In order to improve the flowability and cleanability (the characteristic to be easily cleaned), in some cases, inorganic fine particles or organic fine particles may be added on the surface of the toner particle as necessary.
In recent years, with the rapid development of today's sophisticated information society, there are increasing demands for the ability to provide high image quality information documents constructed in various methods. To address these demands, currently, significant efforts are devoted to researches for improving image quality in various image forming methods. This trend is also true for the image formation through electrophotography, and, in particular, in order to realize a higher resolution image in a color image formation in electrophotography, efforts have been made for data processing techniques of images read by scanners and improvements in digitization techniques in writing with laser. In addition, efforts have been made in research and development of techniques for obtaining smaller size toners, for shaper (narrower) particle size distribution of toners, and for spherical toner particles. For example, when an image is formed using toners having a wide particle size distribution, the toners having a smaller particle size in the particle size distribution cause significant problems such as contamination of developing roller, charging roller, charging blade, photoreceptor, carrier, etc. and spreading of toners. Because of this, it becomes difficult to simultaneously achieve high image quality and high reliability. Such a toner having a wide particle size distribution is also disadvantageous in that reliability is low in a system having functions such as a cleaning function or a toner recycling function. With regard to obtaining a spherical toner particle, the shape of the toner particle significantly affects precision transferability of the toner particle in the transfer stage. That is, the precision transferability becomes higher as the shape is more spherical because the contact area between the toner and the carrier can be maintained at the minimum until the final image is obtained, resulting in a possible improvement in the final image quality characteristics such as reproducibility of fine lines. As described, in order to simultaneously achieve high image quality and high reliability, it is necessary to reduce the particle size of the toner, sharpen the particle size distribution of the toner to obtain a more uniform particle size and a more spherical shape for more uniform surface conditions.
However, because the spherical toners have more uniform surface structures in comparison to randomly-shaped toners such as toners obtained through a grinding process, the distribution of adhering strength with the carrier is narrow, and because the particle size distribution has a narrow width, the distribution of forces applied to the individual toner by a developing electric field has a narrow width. Because of a combination of these two characteristics, as shown in FIG. 1 which shows relationships between the electric filed and amount of development, the spherical toner has a tendency that the developing amount rises, with respect to the developing electric field, in a sharper manner compared to the relationship in randomly shaped toners obtained by grinding. In today's digital color copiers, a latent image is formed by with a laser. In a solid section, the entire surface is exposed and a uniform potential pattern is formed in a wide area. On the other hand, in a halftone region where the image density is lower, the exposure of the laser beam is controlled so that the writing process is performed in a dot-shape or line-shape in order to from a pixel having a very small area of 1 mm2 or less. In addition, the density of the pixel (hereinafter referred to as “input coverage” in the specification) is controlled to reproduce a halftone. The pixels are uniformly exposed with the potential distribution on the photoreceptor and profile of the developing electric field varying depending on the input coverage, that is, in a highlighted section wherein the input coverage is low, the developing electric field is smaller. This does not cause a problem in developing in a region wherein the developing is saturated with respect to the developing electric field such as the solid section and a region of high input coverage. However, in halftone dots having low input coverage, the developing electric field becomes smaller than that for the solid section, and, in some cases, the developing electric field falls out of the saturation range in the electric field-development curve. The electric field felt by the toner depends on the input coverage and becomes lower as the input coverage becomes smaller. Because of this, when a toner having a spherical shape and a narrow distribution in which the density more sensitively responds to the developing electric field is used, in comparison to a ground toner having a wider distribution and random shape, the probability of inability to develop and reproduce in a region of low input coverage is greater. As a result, it is difficult to uniformly develop both the solid section and a region of low input coverage using the spherical toners, and, in some extreme cases, the reproducibility becomes inferior for pixels having an input coverage value smaller than a certain input coverage value. In addition, in some cases, the developing electric field may vary when the distance between the developing roller and the photoreceptor varies due to, for example, deviation of the center. If the developing electric field varies in development of images having an input coverage value of 50 or less on the overall surface, unevenness in images becomes more significant with spherical toners having a sharper development curve with respect to the developing potential compared to ground toners with random shapes. This phenomenon becomes particularly noticeable when the distance between a developing sleeve and photoreceptor (DRS) becomes narrow and the magnitude of the center deviation becomes relatively large compared to the DRS, and also when the peripheral speed of the photoreceptor is high such as in a high-speed copier.
To address this problem, various techniques are employed such as, for example, changing the ratio on the positive side and negative side of an alternating developing bias, but these techniques have not proven too effective so far. Solutions such as an increase in the precision of DRS are not preferable because such solutions causes an increase in cost. Therefore, there is presently no satisfactory method yet.
In order to improve image quality and to significantly reduce toner consumption per page, Japanese Patent Laid-Open Publication No. Hei 11-344837 proposes an electrostatic charge developer comprising a spherical toner having an average volume particle size of approximately 1 μm to 6 μm and a resin-coated carrier having an average volume particle size of 20 μm to 150 μm. In order to obtain high image quality and high image density even when a small size toner is used, Japanese Patent Laid-Open Publication No. 2001-147552 proposes an electrostatic charge developer comprising a toner made by fusion within water-based medium and a resin-coated carrier having a resistivity of 103 Ω·cm–1012 Ω·cm. In this reference, the resistivity of the resin-coated carrier is adjusted by the thickness of the coating layer.
Therefore, an advantage of the present invention is that a developer and an image forming method are provided wherein image quality such as reproducibility of fine lines are improved using spherical toners having a sharp particle size distribution and small particle size while eliminating reproduction deficiencies in low input coverage sections which is a disadvantage of spherical toners and wherein density unevenness can be inhibited even in a document containing entirely halftone images.